Cams are the secret ingredient in the internal
combustion soup...bumpsticks instead of breadsticks! More dangerous
sounding names have been assigned to cams than any other part you'll
find in your motor, further compounding their mystery by giving them
personalities. Who would dare to ask the "Dominatrix"
that lifts your valves what her stats are? We've put together a
calculator that will tell you what your cam's lobe centers and
durations are to shed some light on this mysterious subject.

Open and Shut Case

When does that valve open anyway? Well, to determine
this we have to put dial indicator on the valve to see when it lifts
off of its closed position...for both the inlet and exhaust valves. We
need a reference point for this and we'll choose the engine's Top Dead
Center (TDC) on the particular cylinder we are measuring. TDC is the
point were the piston is as close to the cylinder head as possible and
the place in the engine's rotation where both inlet and exhaust valves
are closed. You'll know they are closed if both pushrods, in the case
of a 2 valve motor, are "loose" and not under tension.

Where the Hell is TDC ?

How do we know exactly where this TDC point is
anyway? Well, to do this we need to put a circular degree wheel on the
end of the crankshaft that has 360 marks, one for each degree of
crankshaft movement as well as marks for TDC (zero degrees) and BDC
(180 degrees). BDC is bottom dead center, where the piston is as far
away from the cylinder head as possible. A piece of wire can be
manipulated into place to align up with the TDC mark (zero degrees).

The best way to find the exact TDC is to use an
adjustable mechanical stop that screws into the the spark plug hole.
Put the motor at a point near to where you think TDC is i.e. both
valves are closed and screw in the mechanical TDC stop. Gently screw in
the adjustable portion of the stop till it contacts the top of the
piston and lock it down. From this point you can gently turn the motor
clockwise and counterclockwise until it gently hits this stop. Note
these two degree figures, split the difference, and put the wire on the
new TDC point (zero degrees). Rotate the engine cw and ccw again to
verify that the mechanical stop hits the piston the same number of
degrees before and after TDC (BTDC, ATDC). Adjust the wire you've
arranged as a pointer, if necessary, to point to the TDC mark. You've
now found TDC! Ground zero in your search for truth.

No Tower of Babel

To insure we all speak the same language we have to
agree on the same starting point. In this case it's how far the valve
lifts before we start writing down those degrees. The industry standard
is when the valve lifts .050 inches (fifty thousandths) off it's seated
position. The idea in this is that we will start at an agreed on point
where there is "measurable" flow, the assumption being that there is no
meaningful flow in the first .050" of valve lift. So, get the cylinder
to TDC, making sure both valves are closed and set a dial indicator on
either your inlet or exhaust valve. Set the indicator dial to zero and
slowly rotate the engine and note the degrees at which the valve hits
this magic .050" point. This will be .050" of lift after it opens and
.050" before it closes.

TDC, BDC, BTDC, ATDC, BBDC, ABDC, Hike!

Since we all speak the same language on this
monumental project we know that BTDC is "before top dead center" and
ATDC is "after top dead center". BBDC is "before
bottom dead center" and logically ABDC is "after bottom dead center".

When you write down the figures you have to add these
acronyms to your degree figures. Inlet cams will generally open x
degrees BTDC and close x degrees ABDC. Exhaust cams will generally open
x degrees BBDC and close x degrees ATDC.

Once you've got your figures at .050" lift for both
the inlet and exhaust valves you can plug these figures into the
calculator to get information about your cam's duration and installed
centerlines.

Sample of Harley-Davidson TC Specs

Manufacturer/Product

Intake Open BTDC

Intake Close ABDC

Exhaust Open BBDC

Exhaust Close ATDC

Andrews TW37

12

42

48

12

Andrews TW64G

30

62

68

32

Crane HTC-290-2

18

42

46

22

Crane HTC-304-2

25

49

56

24

Leinweber RE-T3S

17

41

51

11

Leinweber RE-T5S

24

40

56

12

Red Shift 577TC

25

46

53

22

Red Shift 657TC

21

52

58

16

S&S 640G

25

60

65

25

S&S 675G

25

64

70

25

Screaming Eagle SE-264

24

60

60

22

Sample of Harley-Davidson EVO Specs

Manufacturer/Product

Intake Open BTDC

Intake Close ABDC

Exhaust Open BBDC

Exhaust Close ATDC

Andrews EV51

28

44

54

22

Andrews EV84

32

64

70

30

Crane H290

17

43

45

23

Crane H310

23

63

68

28

Leinweber E-4

49

74

76

39

Leinweber E-9

53

81

86

38

Red Shift 575

25

54

63

18

Red Shift 656

28

52

52

26

S&S 600

20

55

60

20

S&S 631

34

61

66

29

Screaming Eagle SE-60

26

56

61

24

Camshaft Lobe Center / Duration Calculator/Lobe
Separation Angle

Camshaft Name / Number :

Intake Opens BTDC (ATDC is -) :

(in degrees)

Intake Closes ABDC :

(in degrees)

Exhaust Opens BBDC :

(in
degrees)

Exhaust Closes ATDC (BTDC is -) :

(in
degrees)

RSR
Calculations

Overlap, Close the Barn Door

The amount of time, expressed in crankshaft
degrees, that describes the window of time between the the Inlet Cam's
opening point BTDC and the Exhaust Cam's closing point ATDC. This
figure can vary between zero degrees on some stock cams to as much as
70 to 90 degrees on some race motors. In general most street engines
will have 20 to 30 degrees of overlap and most performance cams will
have 50 to 60 degrees of overlap. Increasing the degrees of overlap
tends to move the powerband up the RPM band. Increasing the overlap can
increase peak power, but only if the exhaust system is properly
designed to scavenge the cylinder. Decreasing the overlap tends to
boost lower rpm performance.

Lobe Separation Angle (LSA)

This is the angle between the inltake and exhaust
camshaft lobe peaks described in camshaft degrees. Generally speaking
the majority of cams will fall between 98 and 120 degrees. This angle
dictates two important events: the valve overlap around TDC, and intake
or exhaust valve closure delay there is in the relevant stroke
(inlet/exhaust). Tightening the lobe center angle produces more overlap
around TDC and wider angles mean less overlap.

Lobe
separation angles in turbochargers are used to be around 112-114
degrees but should be less with more modern faster spinning turbos that
have less exhaust back pressure. With less exhaust backpressure LSA's
of 109 dgrees are more the norm. The LSA for turbochargers
is highly dependent on the exhaust backpressure present in the system
design and the turbocharger chosen. More restrictive A/R ratios in the
exhaust scroll of the turbo, with attendant increase in backpressure in
the exhaust, calls for an increase in the LSA to prevent dilution of
the inlet charge.

Valve Lift

A little appreciated consideration is the effect of
valve lift on engine performance. As the engine speed increases there
will be a need to increase valve lift to keep the inlet speeds from
exceeding the Mach Index value of .6, beyound which volumetric
efficiency falls off. This leads to the intriguing possibility of
planning your valve lift in advance relative to your design or
performance goals be they street or racing. Check out our Mach Index Inlet Valve Calculator.

Advanced Dynamic Compression Calculator

When you are planning an engine you need to
understand the relationships that exist between cam timing, static
compression, rod length, bore, stroke, altitude and in the case of RB
Racing Turbos, boost pressure. We have created a calculator that lets you play around with
design parameters and have a good idea of how the engine combination
will work out for your intended usage. By focusing on the dymanics we
avoid the trap of looking at fixed or static values which can often be
misleading. Plug and play before you buy and try!